1. Field of the Invention
This invention relates to a desensitizing solution applied to a sensitized lithographic plate having a layer of photopolymerizable composition.
2. Description of the Prior Art
Among many types of currently available sensitized lithographic printing materials, a lithographic printing plate using a photopolymerizable composition as a photosensitive layer is known to have a long running impression and therefore can be used for printing a large number of sheets. One example of the photopolymer conventionally applied to a lithographic plate is a photosensitive material which mainly consists of a photodimerizable sensitive resin, for instance, a combination of a polyester of p-phenylene diacrylate and 1,4-cyclohexane diol (described in Belgian Pat. No. 696,533), or a reaction product of phenoxy resin and cinnamic acid (described in U.S. Pat. No. 3,387,976), and a polymerizable compound having an ethylenic unsaturated bond, and a binder. Examples of the combination of the ethylenic unsaturated compound with binder are described in U.S. Pat. No. 3,043,805, which include esters such as diethylene glycol diacrylate, triethylene glycol dimethacrylate, and pentaerythritol triacrylate (as the ethylenic unsaturated compound) plus resins such as methyl methacrylate/methacrylic acid copolymer and styrene/itaconic acid copolymer (as the binder). A photosensitive lithographic printing material coated with such a photopolymerizable material is imagewise exposed through a negative pattern and developed by a developer such as an organic solvent or an aqueous alkali which dissolves and removes the unexposed area.
Recently, a dry processable recording material has been proposed as an alternative to the above-described printing material that is processed by a liquid developer. Japanese Patent Publication Nos. 9663/63 and 22901/68 (U.S. Pat. No. 3,353,955) as well as Japanese Patent Application (OPI) Nos. 7728/72 (U.S. Pat. No. 3,770,438) (The term "OPI" as used herein refers to a "published unexamined Japanese patent application") and 46315/75 describe such a new recording material which makes use of the photopolymerization of a photopolymer and the resulting change in adhesive strength. To be more specific, this type of recording material comprises a plastic, metal or paper substrate which is coated with a layer of photosensitive composition containing a polymer as binder and an initiator of photopolymerization of unsaturated monomer, said layer being further overlaid with a cover of a thin transparent film. After imagewise exposure with the original placed on the cover, the cover is peeled off to have the exposed area (or unexposed area) of the photosensitive layer left on the substrate and the unexposed area (or exposed area) left on the cover so that a negative and a positive image (or positive and negative image) are formed on the substrate and cover, respectively. A photosensitive lithographic printing plate using a photopolymerizable composition in such peelable photosensitive material is described in Japanese Patent Application (OPI) No. 9501/77 and Japanese Patent Application No. 66353/76.
A lithographic printing plate is usually subjected to desensitization for rendering the non-image area hydrophilic. Desensitization (1) completely removes the photosensitive layer which was not removed by development, (2) enhances the hydrophilicity of the non-image area by chemical treatment of the metal surface, and (3) further enhances the hydrophilicity of the non-image area by adsorption of hydrophilic colloidal particles. Functions (1) and (2) are accomplished by cronak treatment, phosphate treatment, nital treatment or zirconium salt treatment. Function (3) is generally achieved by coating with gum arabic. To prevent reaction between the photosensitive layer and the substrate, a presensitized plate is generally subjected to anodization of the aluminum substrate or chemical treatment with zirconium fluoride, or with sodium silicate as described in U.S. Pat. No. 2,714,066. Such treatments are effected prior to coating of a photosensitive solution, and after development, an aqueous solution mainly consisting of gum arabic is used to gum the plate for desensitization.
While the primary purpose of gumming is to protect the hydrophilicity of the non-image area, it also has the effect of preventing sebum secreted by fingers, other greasy matter and dust from staining the prepared plate during storage until its use for printing as well as preventing oxidation from occurring during temporary shutdown of a printing machine.
In recent years, there has been a demand in the printing industry for a printing plate of longer running impression. For this purpose, the entire surface of a lithographic printing plate having a photopolymerizable sensitive layer is irradiated with actinic rays after its imagewise exposure and development. Generally referred to as post-exposure or re-exposure, this treatment promotes the polymerization of the image area until it is completely hardened.
However, with a lithographic printing plate that makes use of the photopolymerization of a photosensitive layer which is to be developed by a liquid developer, it is impossible to remove the unhardened area completely by development. In addition, if the developer has been used to process a number of printing plates, the photopolymerizable compound released will be adsorbed on the non-image area and, upon post-exposure, will be polymerized to give a stained non-image area during printing. Likewise, development of a printing plate that uses a peelable cover causes a trace of photopolymerizable composition to be left on the non-image area, and this composition will be polymerized upon post-exposure to give a stained non-image area during printing.